1. Field of the Invention
The present invention relates to a fuel cell in which a plurality of unit cells are stacked, and more particularly to a fuel cell in which reliability of a gas seal performance of a manifold portion is improved and productivity and interchangeability of unit cells are also improved.
2. Description of the Related Art
As is well known, a "fuel cell" is a system for convening Gibbs free energy components directly into electric energy from a difference between a chemical potential of a fuel gas and an oxidant gas and a chemical potential of a product through their reaction.
For instance, in a fuel cell for convening chemical energy directly into electric energy, a pair of electrodes (i.e., an anode and a cathode) are arranged to clamp therebetween an electrolyte tile (layer) containing a molten carbonate as an electrolyte. In its basic operation, a fuel gas and an oxidant gas are separately supplied from both electrodes and are brought into contact with both electrodes to cause a reaction in the cell by shifting ions in the electrolyte. The flow of the ions in the electrolyte is tapped to the outside to obtain an electromotive force.
However, the electromotive force obtained by a unit cell is below 1 V at best. Therefore, in use as a power generation system having a high output, for the providing a large capacity, a fuel cell area is increased (for example, 1 m.times.1 m) and a plurality (for example several tens to about a hundred) of unit cells are stacked in series so that a fuel cell stacked structure is formed. The system is operated while electromotive pans are kept at a high temperature of about 600.degree. to 700.degree. C. to thereby obtain the total output of the respective unit cells.
An example of a conventional fuel cell will now be explained with reference to the drawings.
FIG. 16 is an exploded perspective view showing a primary pan of the conventional fuel cell laminate structure and FIG. 17 is a cross-sectional view taken along the line A--A of FIG. 16 and additionally shows portions of adjacent stacked unit cells.
A separator element 5 which is composed of separator members 5a, 5b and 5c is disposed on each primary surface of an electrolyte tile (i.e., electrolyte layer) 1 confronting with a unit cell 3 composed of an anode 2a and a cathode 2b through each collector plate 4a, 4b.
Also, a dielectric manifold ring (made of, for example, ceramic) 6 is sealingly connected to and laminated on the associated separator element 5.
In general, the separator members 5a, 5b and 5c are made of stainless steel in view of the demands of workability, heat-resistance and an anti-corrosion property against the electrolyte. In general, one side surface partitioned by the separator member 5a forms a fuel gas flow 7a (or an oxidant gas flow 7b) of a first unit cell 3 while the other side surface partitioned by the separator 5a forms an oxidant gas flow 7b (a fuel gas flow 7a) of a second unit cell 3'. The separator element 5 also serves as a fuel gas channel 7c (or an oxidant gas channel 7d) in a stacking direction and has a through holes 8 extending in the thickness direction in fluid communication with manifold rings 6. Since the respective separator elements 5 have to be prevented from short-circuiting with each other due to a mechanical contact, a dielectric property is required for the manifold rings 6 which are mechanically connected to the respective separator elements 5.
Also, it is important to maintain a sufficient spacing for the fuel gas channel 7c and the oxidant gas channel 7d so that the associated gas may be supplied or discharged through the respective channels. Namely, the supply of the fuel gas and oxidant gas to be required for power generation is attained in a bi-directional manner, i.e., in the stacking direction of the unit cells 3 and the interface direction of the unit cells 3. The manifold rings 6 are formed in the stacking direction, whereas the separator elements 5 are formed in the interface direction for the supply through the gas channels 7c and 7d. Then, in order that a gas-tight seal should be kept between the manifold rings 6 and the separator elements 5, in general, the connecting parts for these components are subjected to bonding with brazing 9a or high temperature adhesives or any other suitable sealant process.
As described above, in the fuel cell stack structure (i.e., stack type fuel cell), in general, the manifold rings are made of ceramic and the separator members 5a, 5b and 5c are made of metal. Accordingly, there is a large difference in thermal expansion coefficients between these two components.
As a result, during operation of the fuel cell at times when the temperature increases or decreases, thermal stresses are generated in the manifold rings 6, the separator elements 5 and the brazing material 9a or bonding material such as adhesives. In the case where the generated thermal stresses are large, the separation of the brazing material 9a or adhesives or the fracture of the manifold rings 6 can occur. Also, even in case of a medium amount of thermal stress, due to the accumulation of damage caused by the thermal stresses concomitant with the repeated temperature cycling, the separation or fracture of the brazing material 9a or adhesives can occur or the gas-tight integrity of the manifold portions would degrade.
If, the gas-tight integrity degrades, a degradation of the cell performance may occur due to the entry of the purge gas into the fuel gas and oxidant gas. Also there would be risk of combustion due to the mixture of the fuel gas and oxidant gas. This causes problems in operating reliability and safety. Furthermore, in the assembling work and the manufacturing work of the fuel cells, since the stacked separator elements 5 are bonded through the manifold rings 6, the step for stacking and arranging the unit cells 3 and 3' and the separator elements 5 would be complicated. In addition, after the stacking and arrangement, if a part of the unit cells 3 and 3' suffers any faults, it would be difficult to mount/dismount and interchange the unit cells, which leads to the problem in productivity.